Method of lifting objects of magnetizable material and a system for carrying the method into effect



y 29. 1 J- MARCHER 3,445,105

. METHOD OF LIFTING OBJECTS OF MAGNETIZABLE MATERIAL AND A SYSTEM FOR CARRYING THE METHOD INTO EFFECT Filed lay 9, 1967 Sheet of 2 O M aVoZt /2 Volt m 4 III! |||ll| 40 /01 w I a 7 f T 40 l I k WN WFW mm J, z ll va m ||i 7 m 7 z z Z 5 IW I fi mm J v /T ||||l| H t H l Tl i 3% w A lxlwjyl V J |.L.v m l I I [-Iflo HH A .I litilll lY lm |||I|..'|l|llu6/ I I u n a lLw. w|lalll1wwrllll H K L 6 J m m MML/ W PM M INVENTOR: JD'RGE-N MARC H ER Filed May 9, 1967 May 20, 1969 I J. MARCHER 3.445.

METHOD OF LIFTING OBJECTS 0F MAGNETIZ-ABLE MATERIAL AND A SYSTEM FOR CARRYING THE METHOD INTO EFFECT Sheet 2 of 2 zyi i JfiRGEN MAR 'R United States Patent 4, 39 Int. Cl. B65h 3/16; H01h 47/00 U.S. Cl. 271-18 16 Claims ABSTRACT OF THE DISCLOSURE One or more lifting magnets for objects of magnetizable material, preferably a pile of iron sheets, and in which the current supply is cut off for a brief period in order to separate the uppermost sheet from a number of other, underlying sheets adhering to it. A voltage impulse induced in the winding of the lifting magnets by the fall of the sheet acting in conjunction with the remanence of the sheet and the lifting magnet is fed to an electronic control unit which again cuts in the current to the lifting magnet for permanently retaining the uppermost sheet.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to a method of lifting and conveying objects of magnetizable material, preferably iron sheets, by means of at least one energizable lifting magnet and in which the current supply to the winding of the lifting magnet is cut off for a brief period in order to separate the uppermost sheet from a number of other, underlying sheets adhering to it, the current being subsequently cut in permanently to retain the said sheet during the movement of the lifting magnet to the destination of the sheet, after which the current is cut out again, by which the sheet is released and drops onto its destination.

Description of the prior art Methods of the aforesaid kind are known, but have the drawback that the brief interval of current interruption has to be carefully adapted to the thickness, dimensions and weight of the sheets. It is furthermore difficult to separate sheets adhering to each other owing to a film of water that has penetrated into the space between the sheets and in certain cases even may be frozen to ice. It has been tried to eliminate this drawback by providing the lifting magnet with specially formed pole shoes so that the magnetic field does not penetrate very deeply into the sheets, but also with this measure it is necessary to make allowance for the sheet thickness.

Summary of the invention It is the object of the invention to devise a method by which the said drawbacks are eliminated, and an essential feature of the invention is that avoltage impulse induced in the winding of the lifting magnet by the fall of the sheet (caused by the brief current interruption) acting in conjunction the remanence of the sheet and the lifting magnet,

is fed to an electronic control unit which, after a predetermined brief first time interval T again cuts in the current to the winding of the lifting magnet for permanently retaining the uppermost sheet.

As a result, the re-supply of the current to the lifting magnets will chiefly influence the uppermost sheet which is lying close to the lifting magnets and which is therefore more retarded in its downward movement than the other "ice sheets the inertia of which will aid in separating them from the uppermost sheet. Experience has disclosed that when the time interval T is made sufficiently brief so that it is possible to catch the sheet almost at the moment it is going to drop, a satisfactory separation is obtained even in case of widely varying thicknesses and weights of sheets.

In one embodiment of the method according to the invention the control unit again cuts out the current to the winding of the lifting magnet after a predetermined second time interval T by which a voltage impulse is again induced in the winding owing to the aforesaid remanence. The said voltage impulse is then transmitted to the control unit which again after the time interval T cuts in the current, the said process being preferably automatically repeated a predetermined number of times so as to ensure that all the sheets, except the uppermost one, have been dropped prior to the lifting magnet moving to the destination of the uppermost sheet.

As a result, the sheets are subjected to repeated mechanical action owing to their inertia so that they are shaken from each other and any adhesion between them is gradually and finally destroyed, whereas the uppermost sheet is securely retained after each of the mechanical actions.

In order that the brief time intervals T necessary to catch the sheet may be created before the sheet has fallen such a distance that the lifting magnet is no more able to reattract it, it has been found necessary to use electronically controlled rectifiers for cutting in and cutting out the current through the winding of the lifting magnet.

The invention is furthermore concerned with a system for carrying the method according to the invention into effect and with at least one energized lifting magnet which may be moved vertically by a hoisting mechanism, and an essential feature of a system according to the invention is that the winding of the lifting mechanism is connected via an electronic control unit to a trigger circuit for a controlled rectifier unit.

As a result, the moments of cutting in and cutting out the current to the lifting magnet may be determined at great accuracy, even though the time interval between the said cutting-in and cutting-out operations is of the order of fractions of a second.

In one embodiment of the system according to the in vention there is inserted a diode transversely across the input terminals of the lifting magnet, the said diode being connected in series with the primary winding of a current transformer, and the outer terminals of the secondary winding of the current transformer are connected to input terminals of the control unit.

\ As a result, a voltage impulse is transmitted through the current transformer to the control unit when the current in the winding of the lifting magnet is interrupted, and further the excess voltages produced by the said interruption of current in the winding of the lifting magnet are substantially reduced, being equalized through the diode and the low-impedanced primary winding of the current transformer.

According to the invention the control unit incorporates a first regulable time measuring unit which is provided with an electronic output relay and the input of which is connected via a flip-flop unit to a sensing contact mounted on the yoke of the lifting magnet, the said sensing contact being so mechanically controlled that when the yoke is resting in contact with the upwardly facing surface of a sheet stack, the contact is closed, and, when the yoke is lifted somewhat above the said surface, the contact is open, and the output relay of the time measuring unit is connected to the flip-flop unit in such manner that the latter is set back after a predetermined time interval T during which the hoisting of the yoke is continued.

As a result, the lifting magnet is lifted a further amount beyond the predetermined amount that is necessary to break the sensing contact. The said further amount may be varied so by a regulation of the time interval T that the sheet or the sheet stack receives a fixed free height of fall, for example of 20-30 cm.

According to the invention the output relay of the first time measuring unit is furthermore connected to a second flip-flop unit, one output of which is connected with an amplifier with a contactor the breaker contacts of which are inserted in series with the current supply to the motor of the hoisting mechanism, as a result of which the motor of the hoisting mechanism is cut out at the end of the time interval T According to the invention the other output of the second flip-flop unit is connected to an input of a control amplifier, the output terminal of which is connected to the trigger circuit, and the said second fiip-fiop unit blocks across its connection to the electronic relay in the first time measuring circuit after the time interval T H a control voltage fed from the said relay to the said input of the control amplifier.

As a result, the controlled rectifier stops its ignition impulses and cuts oil the current to the lifting magnet, after which the stack of sheets begins to fall.

According to the invention the one output of the second fiip-flop unit is furthermore connected to a second regulable time measuring unit which is provided with an electronic output relay connected to the second flip-flop unit in such manner that the latter is set back after a predetermined time interval T for cutting out the control rectifier and its relay.

As a result, the hoisting mechanism is only re-started after the end of the time interval T and the said time interval may be regulated to predetermine a moment for shaking off or separating the underlying sheets, after which moment only the uppermost sheet is retained by the lifting magnet.

According to the invention one output of the second flip-flop unit is further across a first AND gate connected to a third, regulable time measuring unit which is provided with an electronic output relay connected in such manner through a lead to a third flip-flop unit that the latter is set back and through another lead prepares the opening of a second AND gate for a voltage impulse which is generated across the input terminals of the control unit when the controlled rectifier is cut out.

As a result, the control unit is blocked during the vibrations and swinging-phenomena which may arise directly after the current to the lifting magnet being interrupted to prevent the said current from being cut in too early, the said cutting-in operation being only possible after the elapse of a time interval T According to the invention the input terminals of the control unit are connected via an impulse amplifier and an electronic relay to an input of the second AND gate. As a result, the voltage impulse from the winding of the lifting magnet opens the second AND gate.

According to the invention the output of the second AND gate is connected through a fourth flip-flop unit to a fourth regulable time measuring unit which is provided with an electronic output relay that is connected through a lead and a third AND gate to the control amplifier in such manner that the output terminal of the said amplifier after a predetermined time interval T is fed with a trigger voltage for the controlled rectifier.

As a result, the controlled rectifier will, after the elapse of the time interval T again out in the lifting magnet so as to arrest the fall of the stack of sheets and to shake off one or more of the sheets adhering to the uppermost one.

According to the invention the output of the second AND gate is furthermore connected through a fifth flipfiop unit to a fifth regulable time measuring unit having an electronic output relay which is connected through a 4, lead, partly to the other output of the second flip-flop unit for setting back the fifth time measuring unit after a fixed time interval T and partly to the third flip-flop unit of the third time measuring unit.

As a result, the current supply to the lifting magnet is again interrupted by the controlled rectifier after the predetermined time interval T by which another voltage impulse is generated by the winding of the lifting magnet, and, further, the third time measuring unit is again out in so as to block the influence of sheet vibrations and swinging-in phenomena upon the moment of renewed cutting-in of the lifting magnet. A new period in the process of shaking off and separating the sheets has thus been commenced, the said period being only finally interrupted when the second time measuring unit has measured the predetermined total time of shaking off and separating the sheets.

The invention will now be further described with reference to the drawing, in which FIGURE 1 shows a wiring diagram of a current supply unit for a system according to the invention, and

FIGURE 2 a wiring diagram for an electronic control system for same and FIGURE 3 a simplified time diagram of the operation of the system.

In the drawing, 1 denotes one or more lifting magnets mounted in a crane (not shown) for lifting and conveying objects of magnetizable material, preferably iron sheets, the said lifting magnet being energized by a threephase supply mains R, S, T through rectifiers 2, 3 and 4 with a load resistor B and thyristors 5, 6 and 7 the control electrodes 8 of which are connected through a resistor 9 to a trigger circuit 10 which at moments determined by an electronic control unit 11 cuts in and cuts out the thyristors 5-7. The control unit 11 is fed with current from the supply mains via terminals R and O and transmits voltages minus 12 volts and 0 v. to the trigger circuit 10 via terminals 12 and 13 at the predetermined moments, after which the trigger circuit lights and extinguishes the thyristors 5, 6 and 7, thus cutting in and cutting out, respectively, the current from the rectifiers 2, 3 and 4 to the lifting magnet 11.

In parallel across the input terminals 12 and 13 of the lifting magnet is connected the primary winding 15 of a current transformer 14 through a diode 16, which serves to protect the rectifiers 2, 3 and 4 against excess voltages generated in the winding of the lifting magnet when the current through same is interrupted, such excess voltages being short-circuited by the diode 16 which further ensures that only current impulses 17 at a fixed polarity may pass through the primary winding 15. The said current impulses 17, indicated in FIGURE 3, are used for activating the control unit 11 and they are generated when the current through the lifting magnet 1 is interrupted, since at this moment, when a sheet or stack of sheets (not shown) lifted by the lifting magnet 1 begins to fall, a voltage impulse 17 will be induced in the winding of the lifting magnet owing to the remanence of the iron sheet or the stack of iron sheets and in the yoke or the lifting magnet. The trigger circuit 10 contains a generator of impulses of relatively high frequency such as 2000 Hz.; the said impulses are capable of activating the thyristors once every half millisecond, and the mo ment of interrupting and cutting in again currents to the lifting magnet is determined by the voltage impulse 17 with an accuracy of less than 0.5 millisec. The interruption of the current to the lifting magnet starts a periodically repeated process continuing for a fixed period within which all the sheets except the uppermost one, which is retained, are shaken off.

The process of shaking ofl? or separating the sheets is illustrated in its sequence by means of the time diagram of FIGURE 3. At the moment t a hoisting mechanism (not shown) in which the yoke of the lifting magnet is suspended begins lowering the lifting magnet towards a sheet stack (not shown). The time t indicates that the magnet has struck the uppermost sheet of the stack and at this moment a contact 18a disposed on the yoke of the lifting magnet causes via contactors (not shown) a reversion of the current I to the hoisting mechanism, as a result of which the contact 18a is opened when the yoke with the sheet stack retained by the lifting magnet has been slightly lifted, the said interruption by the contact 18a indicating that one, or more sheets are being hoisted. The yoke of the lifting magnet is further provided with a contact 18b inserted, as shown in FIGURE 2, between a zero terminal 19 and an input terminal 20 of the control unit 11, the said input terminal being connected via a drop resistor 21 to a direct current terminal 22 of, say, 12 volts. When the contact 18b is opened at the moment t the potential of the input terminal 20 is suddenly changed from 0 volts to -12 volts by which a regulable time measuring unit TH is started, and hoisting is continued for a period T which is fixed by the adjustment of the time measuring unit TH and determines the height to which the sheet stack is to be lifted before the lowermost sheets of the stack are to be shaken off. The time measuring unit TH which is cut in via the first flip-flop unit 23 consists of an electronic relay 24 and a charging circuit constituted by a capacitor 25 which is earthed through the frame M and a regulable resistor 26. After the period T the flip-flop unit 23 is set back by the relay 24, this interrupting the time measuring unit TH at a moment 1 Simultaneously a second flipfiop unit 27 is activated, and via an amplifier 28 and a contactor 29 having terminals 30, 31 inserted in series with the motor of the hoisting mechanism the said flipflop unit 27 stops the hoisting via its breaker contacts 32. Furthermore, a second regulable time measuring circuit TSK is activated. The said circuit TSK is composed of an electronic relay 33 and a charging circuit which is constituted by a capacitor 34 earthed through the frame M and a regulable resistor 35. The second time measuring circuit TSK determines a total time T for shaking off or separating the sheets, elapsing at the moment t At the end of the separating or shaking off time T the flip-flop unit 27 is set back by the electronic relay 33 of the time measuring circuit TSK. Simultaneously with the start of the shaking off time, that is, at the moment t an input voltage of a control amplifier ST is interrupted, the said control amplifier has at least two inputs 36, 37, one of these 36 being connected to a flip-flop unit 27 and having at one output a terminal 38 which is also input terminal for the trigger circuit and the other output of which is connected to a pilot lamp 39 which is lighted when the interruption of theinput voltage of the control amplifier ST via the input 36 causes an interruption of the thyristors 5, 6 and 7 and thereby the current I to the lifting magnet 1 is interrupted, after which the sheet stack adhering to the lifting magnet begins to drop.

As soon as this occurs, the voltage impulse 17 would be generated, but since use is generally made of several lifting magnets in practice, in numbers of, say, up to 12, there may occur an interval of uncertainty during which the magnets that are first to release the sheet will give rise to excess voltages, and to ensure that these excess voltages do not cause the lifting magnets to be cut in again too early it is convenient according to the invention to insert in the control circuit 11 a time delay circuit consisting of a third regulable time measuring circuit TBL to fix a period T during which the control amplifier ST is blocked, that is, until the moment t The time measuring circuit TBL consists of an electronic relay 40 and a charging circuit which is constituted by a capacitor 41, which is earthed through the frame, and a regulable resistor 42, and the time measuring circuit is started by a third flip-flop unit 43 via an AND gate 44 connected with its one input 45 through a lead 46 to the second flip-flop unit 27 in such manner that the AND unit 44 is opened when the flip-flop unit 27 starts the separating time T at the moment t After the period T that is, at the moment t the third flip-flop unit 43 is set back by the electronic relay 40 via leads 47, 48 and an AND gate 49, by which a signal is transmitted from the said flip-flop unit 43 through a lead 50 to a first input 51 of a second AND gate 52.

The control unit 11 is subsequently ready to detect the voltage impulse 17 generated in the lifting magnets when the sheet stack begins to fall after the main part of the lifting magnets having released the uppermost sheet. The secondary winding 53 of the current transformer 14 is as indicated at the upper right-hand side of FIGURE 2 connected through its outer terminals 54 and 55 to a potentiometer 56 of the control unit 11, from which potentiometer there is obtained a regulable signal which is transmitted via an impulse amplifier 57 and an electronic relay 58 to an input 59 of the second AND gate 52, the third and fourth inputs of which, 60 and 61 respectively, are connected to 12 volts and to a first input of a third AND gate 62, respectively. When the voltage impulse 17 is generated, the input 59 will likewise receive l2 volts for a brief interval, and the said voltage is transmitted through the second AND gate 52, which has now an active signal on all its inputs, to the output 63 of the AND gate, by which a fourth and a fifth flip-flop unit 64 and 65 are acted upon in such manner that both receive 12 volts for their outputs 66 and 67, respectively. As a result, a fourth and a fifth time measuring unit, TRE and TIN, respectively, are started. The time measuring unit TRE, which consists of an electronic relay 68 and a charging circuit constituted by a capacitor 69 earthed through the frame and a regulable resistor 70, determines the duration of the time interval T to elapse between the voltage impulse 17 and the reactivation of the lifting magnets 1 for arresting the fall of the sheet stack. The time measuring unit TIN, which consists of an electronic relay 71 and a charging circuit constituted by a capacitor 72 earthed through the frame and of a regulable resistor 73, determines the duration of the time inter-val T to elapse between the voltage impulse 17 and the cutting out again of the lifting magnets, by which another successive voltage impulse 17 is generated.

After the time interval T which is very brief, say, less than 20 msecs., the fourth flip-flop unit 64 is set back by the electronic relay 68 through a lead 74, and an activating signal is transmitted from the output 75 of the said unit 64 to a first input 76 of the third AND gate 62 which further receives an activating signal from the fifth flip-flop unit 65 on another input 77 at the moment t the other inputs 78 of the third AND gate 62 have constantly l2 volts as indicated in the drawing. At the moment i there will therefore be a signal voltage on'the second output 79 of the third AND gate 62, the said output 79 being connected through a lead 80 with the other input 37 of control amplifier ST, and the trigger unit 10 is cut in across the terminal 38, by which a control voltage is fed across the resistor 9 to control the electrodes 8 of the thyristors 5, 6 and 7, by which the supply of current I to the windings of the lifting magnets 1 is resumed.

After the elapse of the time interval T that is at the moment t the time measuring unit TIN will via a fourth AND gate 81 and a lead 82 reset the fifth flip-flop unit 65, by which the activating voltage on the other input 77 of the third AND gate 62 is eliminated and as a simultaneous result the output voltage of its output 79 and the voltages of the other input 37 of the control amplifier ST will also be eliminated so that the control voltage for the terminal 38 of the trigger unit 10 is elimi nated too, by which the current supply I to the lifting magnets 1 from the rectifiers 2, 3 and 4 is interrupted.

Simultaneously, the time interval T of blocking is again started by the third flip-flop unit 43, and after this time interval the control circuit 11 is again ready to detect a fresh voltage impulse 17, after which the process is repeated until the total time T has elapsed, when the electronic relay 43 sets back the second flip-flop unit 27, and via its output and the control amplifier ST and the terminal 38 the flip-flop unit 27 causes the lifting magnets 1 to be permanently cut in.

In order to ensure that the time measuring units TBL and TIN rapidly discharge their respective capacitors 41, 72, diodes 83 and 84 are connected transversely across the regulable resistors 42, 73; the said diodes have such a polarity that they block the direction of the charging voltage, but are conducting for the discharge currents of the respective capacitors.

Tests carried on with plants according to the invention have disclosed that with the use of the following adjusted time intervals:

T abt. 0.5 sec.

T =-20 msec.

T abt. 8 secs.

T abt. 2 secs. and

and a suitable value selected for the input sensitivity of the impulse amplifier 57, it was possible to separate a stack of sheets having thicknesses of 2-8 mm. and to separate another stack of sheets having thicknesses of 8-40 mm. without in the last separation losing the uppermost sheet of the stack.

I claim:

1. A method of lifting and conveying objects of magnetizable material, preferably iron sheets, by means of at least one energized lifting magnet and in which the current supply to the winding of the lifting magnet is cut off for a brief period in order to separate the uppermost sheet from a number of other, underlying sheets adhering to it, the current being subsequently cut in permanently for retention of the uppermost sheet to the lifting magnet during the movements of the latter to the destination of the sheet, after which the current is cut out again, by which the sheet is released and drops onto its destination, characterised in, a voltage impulse induced in the winding of the lifting magnet by the fall caused by the brief current interruption acting in conjunction with the remanence of the sheet and the lifting magnet is fed to an electronic control unit which, after a predetermined brief first time interval T again cuts in the current to the winding of the lifting magnet for permanently retaining the uppermost sheet.

2. A method as claimed in claim 1, characterized in, that the control unit after a predetermined second time interval T again cuts out the current to the winding of the lifting magnet, by which a voltage impulse is again induced in the winding owing to the said remanence, the said voltage impulse being then transmitted to the control unit which again after the time interval T cuts in the current, the said process being preferably automatically repeated a predetermined number of times to ensure that all the sheets, except the uppermost one, have been dropped prior to the lifting magnet moving to the destination of the uppermost sheet.

3. For use in lifting stacked objects of magnetizable material by means of at least one energized lifting magnet wherein the current supply to the winding of the lifting magnet is cut off for a brief period in order to separate the uppermost object from a number of other underlying objects adhering to it, the current being subsequently cut in permanently for the retention of the uppermost object to the lifting magnet during movement of the object to its destination; and electric circuit comprising in combination, an energizable lifting magnet adapted to lift magnetizable objects, current supply means for energizing said magnet, detecting means (14, 15, 16) for detecting a voltage impulse (17) induced when the magnet is deenergized by interruption of the current to the winding of the magnet, the induced voltage being caused by the fall of the objects from the magnet acting in conjunction with the magnetic remanence of the lifting magnet and objects, a trigger circuit (10) controlling the supply means (2, 3, 4 and 5, 6, 7) to the Winding of the lifting magnet, said trigger circuit being adapted to cut in the current from the current supply means to the winding of the lifting magnet after a predetermined first time interval (T for permanently retaining the uppermost object, and means including an electronic control unit (11) interconnecting said trigger circuit and said detecting means whereby the trigger circuit will cut in the current from the current supply to the winding of the lifting magnet in response to detection by the detection means of the voltage impulse induced by the fall of said objects acting in conjunction with said remanence whereby the uppermost object will be retrieved by said magnet for ultimate transportation to its destination.

4. The electric circuit defined in claim 3 wherein said detecting means includes a current transformer having a primary winding (15) and a secondary winding (53), a diode 16 connected in series with the primary winding (15 said series connection being connected to the terminals of the winding of the lifting magnet and adapted to rectify said voltage impulse introduced into said winding and to produce a unidirectional impulse over the terminals (54a, 55a) of the secondary winding (53), said electronic unit (11) having input terminals (54b and 55b), said secondary winding (53) being connected to the input terminals (54b and 55b) of said electronic unit for transferring said unidirectional impulse via said electronic unit to the trigger circuit (10) for actuating said trigger circuit to cut in the current from the current supply means (2, 3, 4 and 5, 6, 7) to the winding of the lifting rnag-.

net after said predetermined first time interval (T 5. An electric circuit defined in claim 4 characterized in, that the control unit 11 incorporates a first regulable time measuring unit TH which is provided with an electronic output relay 24 and the input of which is connected via a flip-flop unit 23 to a sensing contact 18b mounted on the yoke of the lifting magnet 1 and controlled mechanically in such manner that when the yoke is resting in contact with the upwardly facing surface of a sheet stack, the contact 18b is closed, and, when the yoke is lifted somewhat above the said surface, the contact is open, the output relay 24 of the time measuring unit TH being connected to the flip-flop unit 23 in such manner that the latter is set back after a predetermined time interval T during which the hoisting of the yoke is continued.

6. An electric circuit defined in claim 5, characterized in, that the relay 24 of the first time measuring unit TH is furthermore connected to a second flip-flop unit 27, to one output of which is connected an amplifier 28 with a contactor 29, the breaker contacts 32 of which are inserted in series with the current supply to the motor of the hoisting mechanism.

7. An electric circuit as defined in claim 6, characterized in, that the other output of the second flip-flop unit 27 is connected to an input of a control amplifier ST, the output terminal 38 of which is connected to the trigger circut 10, the said second fiip-fiop-unit 27 blocking across its connection to the electronic relay 24 in the first time measuring circuit TH after the time interval T a control voltage fed more from the said relay to the said input of the control amplifier ST.

8. An electric circuit as defined in claim 7, characterized in, that the said one input of the second flip-flop unit 27 is further connected to a second regulable time measuring unit TSK which is provided with an electronic output relay 33 connected to the second flip-flop unit 27 in such manner that the latter is set back after a predetermined time interval T for cutting out the control amplifier 28 and its relay 29.

9. An electric circuit as defined in claim 8, characterized in, that the one output of the second flip-flop unit 27 is furthermore across a first AND gate 44 connected to a third regulable time measuring unit TBL which is provided with an electronic output relay 40 connected in such man- 9 ner through a lead 48 to a third flip-flop unit 43 that the latter is set back and through another lead 50 prepares the opening of a second AND gate 52 for a voltage impulse 17 which is generated across the input terminals 54b, 55b of the control unit when the controlled rectifier 2, 3, 4 and 5, 6, 7 is cut out.

10. An electric circuit as defined in claim 9, characterized in, that the input terminals 54b, 55b of the control unit 11 are connected via an impulse amplifier 57,and an electronic relay 58 to an input 59 of a second AND gate 52.

11. An electric circuit as defined in claim 10, characterized in, that the output 63 of the second AND gate 52 is connected through a fourth flip-flop unit 64 to a fourth regulable time measuring unit TRE which is provided with an electronic output relay 68 that is connected through a lead 74 and a third AND gate 62 to the control amplifier ST in such manner that the output terminal 38 of the amplifier after a predetermined time interval T is fed with a trigger voltage for the controlled rectifier 2, 3, 4 and 5, 6, 7.

12. An electric circuit as defined in claim 11, characterized in, that the output 63 of the second AND unit 52 is further through a fifth flip-flop unit 65 connected to a fifth regulable time measuring unit TIN provided with an electronic output relay 71 which is connected through a lead 82 partly to the other input of a second flip-flop unit 27 for setting back the fifth time measuring unit TIN after a fixed time interval T partly to the third flip-flop unit 43 of the third time measuring unit TBL.

13. Apparatus for engaging and releasing and subsequently engaging a magnetizable object, comprising in combination; an energizable magnet adapted to engage and lift a magnetizable object upon energization of the magnet, and means for subsequently reenergizing said magnet after said magnet is deenergized and in response to the fall of the object from the magnet for reengaging the magnet on the object.

14. The subject matter defined in claim 13 wherein said means includes the magnetic remanence of said magnet and said object as acted upon by the fall of the object and a resulting induced voltage produced thereby, and a detecting means for detecting said induced voltage to thereby control reenergization of the magnet.

15. The method comprising in combination, the steps of; engaging and lifting a magnetizable object by means of an energizable magnet, deenergizing the magnet to cause the object to fall from the magnet, sensing the fall of the object and in response thereto reenergizing the magnet to reengage the object on the magnet.

16. The method defined in claim 15 including the steps of employing the magnetic remanence of the object and the magnet and the fall of the object to induce a voltage for use in reenergizing the magnet.

References Cited UNITED STATES PATENTS EDWARD A. SROKA, Primary Examiner.

US. Cl. X.R. 317-123 

